Four of the five Americans killed when a U.S. security company's helicopter crashed in a dangerous Sunni neighborhood in central Baghdad were shot execution style in the back the head, Iraqi and U.S. officials said Wednesday.

A senior Iraqi military official said a machine gunner downed the helicopter, but a U.S. military official in Washington said there were no indications that the aircraft, owned by Blackwater USA, had been shot out of the sky. Two Sunni insurgent groups, separately, claimed responsibility for the crash.

In Washington, a U.S. defense official said four of the five killed were shot in the back of the head but did not know whether they were still alive when they were shot. The defense official spoke on condition of anonymity because he was not authorized to speak on the record.

The helicopter was shot down after responding to assist a U.S. Embassy ground convoy that came under fire in a Sunni neighborhood in central Baghdad, said a U.S. diplomatic official in Washington.

A second helicopter also was struck, but there were no casualties among its crew, said the diplomatic official, who spoke anonymously because he was not authorized to make statements.

The doomed helicopter swooped into electrical wires before the crash. U.S. officials said it was not clear if gunfire brought the aircraft down or caused its pilot to veer into the wires during evasive manuevers.

The Iraqi official, who also declined to be identified because details had not been made public, said the four were shot in the back of the head while they were on the ground. The crash occurred in an old neighborhood of narrow streets on the east bank of the Tigris River, north of the central city.

In separate fighting Wednesday, U.S. and Iraqi troops battled gunmen firing heavy weapons from concrete high-rises in another Sunni insurgent stronghold, on the west bank of the Tigris north of the heavily fortified Green Zone. Iraq's defense minister said as many as 30 militants were killed and 27 captured.

Apache attack helicopters buzzed past the tall buildings and radio towers along Haifa Street, while several Humvees drove on the tree-lined street below. Gunfire rang in the background as shells fell, according to AP Television News footage.

The U.S. military said the targeted raids were intended to clear the area of militants, dubbing the operation dubbed Tomahawk Strike 11. The clashes were the second major fighting to break out in the area in less than a month.

In the aftermath of Tuesday's Blackwater helicopter crash, U.S. forces were planning to blow up the wreckage to prevent people from scavenging equipment, the Iraqi official said.

Blackwater USA confirmed that five Americans employed by the North Carolina-based company as security professionals were killed, but provided no identities or other details.

On Wednesday, U.S. Ambassador Zalmay Khalilzad offered condolences for the five Americans killed.

"We lost five fine men," Khalilzad told reporters during a round-table discussion at the embassy in the heavily fortified Green Zone in Baghdad.

He said he had traveled with the men who were killed and had gone to the morgue to view the bodies, but offered no further details beyond saying that it was difficult to determine what happened because of "the fog of war."

Another American official in Baghdad, who spoke on condition of anonymity, said three Blackwater helicopters were involved. One had landed for an unknown reason and one of the Blackwater employees was shot at that point, he said.

That helicopter apparently was able to take off but a second one then crashed in the same area, he added without explaining the involvement of the third helicopter.

The Qatar-based Al-Jazeera television said the 1920 Revolution Brigades insurgent group claimed responsibility for shooting down the helicopter and showed a video taken by a cell phone of a mass of still-smoldering twisted metal that it was said was the wreckage of the chopper.

Another Sunni insurgent group, the Ansar al-Sunnah Army, also claimed responsibility and posted identity cards of men who were on the helicopter on a Web site, including at least two that bore the name of Arthur Laguna, who was later identified by his mother as among those killed.

Laguna was a 52-year-old pilot for Blackwater who previously served in the Army and the California National Guard, his mother, Lydia Laguna, of Rio Linda, Calif., told the AP. She said she received a call from her other son, also a Blackwater pilot in Baghdad, notifying her of Arthur's death.

Witnesses in the Fadhil neighborhood told the AP that they saw the helicopter go down after gunmen on the ground opened fire. Accounts varied, but all were consistent that at least one person operating the aircraft had been shot and badly hurt before the crash.

Blackwater USA provides security for State Department officials in Iraq, trains military units from around the world, and works for corporate clients.

"These untimely deaths are a reminder of the extraordinary circumstances under which our professionals voluntarily serve to bring freedom and democracy to the Iraqi people," the Blackwater statement said.

Katy Helvenston, mother of Scott Helvenston, a Blackwater employee who died in March 2004 when a frenzied mob of insurgents ambushed a supply convoy they were escorting through Fallujah, said Tuesday's crash "just breaks my heart."

"I'm so sick of these kids dying," she said.

Before Tuesday's crash, at least 22 employees of Blackwater Security Consulting or Blackwater USA had died in Iraq as a result of war related violence, according to the Web site iCasualties.org, which tracks foreign troop fatalities in Iraq.

The crash of the small surveillance helicopter, believed to be a version of the Hughes Defender that was developed during the Vietnam War, was the second associated with the U.S. war effort in Iraq in four days.

A U.S. Army Black Hawk helicopter went down Saturday northeast of Baghdad, killing all 12 service members on board. The American military in Baghdad has refused to confirm a report by a Pentagon official that debris at the crash site indicated the helicopter was shot out of the air by a surface-to-air missile.

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